The present invention relates generally to nondestructive examination (NDE) techniques used to inspect workpieces for flaws. More particularly, the present invention is drawn to a new and useful method for detecting corrosion fatigue cracks or other flaws in boiler tubes using Electromagnetic Acoustic Transducers (EMATs). The method according to the present invention is particularly suited to the task of detecting flaws in a ferromagnetic component or workpiece such as a boiler tube or a membrane wall tube panel comprised of boiler tubes, even those in close proximity to a probe assembly housing the EMATs.
Corrosion fatigue is a serious failure mechanism for boiler tubes in fossil utility boilers. Mechanical constraints or stress and corrosion work together to produce cracking that is typically oriented axially to the tube axis. This type of damage is usually found close to weld attachments such as buckstay welds. In the past, it has been particularly difficult to locate this type of damage due to the complex geometry of the usual membrane boiler tube panels. Solving the problem of detection of corrosion fatigue is ranked near the top in priorities by the Electric Power Research Institute (EPRI).
As noted, stresses are typically found adjacent the buckstay welds on the casing (cold) side of the furnace wall in a fossil utility boiler. The cracking mechanism may occur in other areas in the boiler; however, the furnace wall is particularly important because the resulting tube failures can lead to a continuing sequence of forced outages. Detection is difficult with ultrasonic techniques (UT) because of the complex geometry of membrane panels. Radiography testing (RT) has been used in the past; however, it is no longer used because of the safety hazard.
U.S. Pat. No. 5,526,691 to Latimer et al. addresses the need for the detection of corrosion fatigue in boiler tubes. The technique of the '691 patent uses a tone-burst pulser to produce horizontally polarized shear waves (SH waves) to propagate past the membrane of a tube panel and detect damage on the casing (cold) side of the furnace wall while the inspection is performed from the fireside. Advantages of that approach include the fact that the casing does not have to be removed, access is easier from the furnace side with stationary or floating scaffolding, and detection of corrosion fatigue cracks from the areas with welded attachments (such as buckstay welds) is more easily accomplished.